1. Field of the Invention
The present invention relates to an improvement in a coating method for the vehicle body of an automobile.
2. Description of the Related Art
In the production of the vehicle body of an automobile, a steel sheet material or an aluminum alloy sheet is press-formed into interior members of the vehicle body such as a floor panel and an inner panel, and the obtained formed members are joined together, as by welding, to produce a chassis which is the skeleton of the vehicle body. Similarly, a steel sheet material or an aluminum alloy sheet is press-formed into exterior members of the vehicle body such as doors, a roof, fenders, a bonnet and a trunk lid, and the exterior members are joined to the chassis. After that, coating is performed to produce the vehicle body.
The surfaces of such steel sheet material and aluminum alloy sheet are coated with a lubricant so that they can easily be press-formed. For this reason, a coating process which follows the press-forming process or the assembly of the chassis generally includes the following steps:
(1) a degreasing and cleaning step (the lubricant on the press-formed steel sheet or aluminum alloy sheet is removed, and in general, an alkali degreaser is used);
(2) a before-coating treatment step (after degreasing, a conversion treatment intended for surface adjustment and an increase in coat adhesion is performed);
(3) an undercoating step (undercoating is performed by electrodeposition coating for the purposes of rust prevention);
(4) an intermediate coating step (the irregularities of the undercoated surfaces are corrected to improve the finish of the following overcoating and to increase chipping resistance); and
(5) an overcoating step (overcoating is performed to give the coated surfaces an aesthetic appearance as well as weather resistance, chemical resistance, wear resistance and the like).
In this manner, the general vehicle body coating method includes a large number of steps and carries out coating along a serial line, so that the coating line is extremely long. In steps such as rust prevention treatment and undercoating, since electrodeposition coating is performed, an immersion coating device or the like is needed. For this reason, large equipment is needed, so that high costs are required for the maintenance and management of the equipment.
The automobile coating method disclosed in Japanese Patent Laid-Open Publication No. HEI-5-115839 has heretofore been known as a coating method which enables an electrodeposition tank and a coating device to be reduced in size.
In this coating method, a chassis is formed from an aluminum alloy, and after the chassis has been subjected to a before-coating treatment, exterior members such as doors, a bonnet, fenders and a roof are subjected to electrodeposition coating followed by finish coating, separately from the chassis.
In this coating method, since the exterior members are subjected to electrodeposition coating followed by finish coating, separately from the chassis, equipment such as an electrodeposition tank and a coating device is reduced in size, and the throwing power of electrodeposition coat is improved and the finish coating is improved.
In Japanese Patent Laid-Open Publication No. HEI-9-131568, a surface-treated aluminum alloy sheet in which a lubricative coat is formed on an aluminum alloy sheet in advance is proposed as a precoat aluminum alloy sheet for a vehicle body constituent material.
The present inventors have made examinations as to whether the coating of a vehicle body can be made simple by developing the coating method of the above-cited Japanese Patent Laid-Open Publication No. HEI-5-115839, and have found out that the coating of a vehicle body can be simplified to a great extent, by noting the above-described precoat aluminum alloy sheet.
Moreover, in order to reduce the weight of the vehicle body of an automobile, particularly, to improve fuel consumption and the like, the present inventors have noted an aluminum-alloy-made vehicle body which has recently been adopted in a wide range of applications, and have made the invention.
An object of the present invention is to provide a vehicle body coating method for an automobile which enables vehicle body constituent members or a coating process to be simplified to achieve easy coating work, thereby reducing a reduction in the entire coating line.
According to a first aspect of the present invention, there is provided a vehicle body coating method for an automobile, which comprises: a step of press-forming a precoat metal sheet in which a rust-preventing layer and a coat which contains a lubricating component are formed on at least an outside surface of a metal sheet, thereby producing plural exterior members, such as a bonnet, a trunk lid, doors and fenders, which constitute an outside of a vehicle body; a step of press-forming a precoat metal sheet in which a rust-preventing layer which contains a lubricating component is formed on a surface of the metal sheet, thereby producing plural inside members, such as a floor panel and an inner panel, which constitute an inside of the vehicle body, and joining the inside members to produce a chassis for the automobile; a step of joining the exterior members to the chassis; and a step of performing intermediate coating and overcoating in order on surfaces of the exterior members joined to the chassis.
In the first aspect of the invention, it is not necessary to perform conversion treatment on the inside members and the exterior members of the vehicle body, and it is not necessary to perform electrodeposition coating required for the chassis. Specifically, it is not necessary to perform any of the step of degreasing and cleaning a press-formed metal sheet, the step of drying the metal sheet, the step of adjusting the surfaces of the metal sheet, a conversion treatment step which performs rust-preventing treatment on the surfaces, and a rust-preventing electrodeposition coating step, all of which steps have been required in the related art vehicle body coating method. In particular, since the exterior members which constitute the outside of the vehicle body need only intermediate coating and overcoating, the coating of the exterior members can be implemented with a spray type of electrodeposition coating device, whereby coating work is made simple and coating equipment can be made small and simple. Drying and fixing equipment for intermediate coating and overcoating can also be made small and simple.
On the reverse surface of each of the exterior members, a rust-preventing layer made of a conversion treatment coat is formed on the metal sheet, and a coat which contains a lubricating component is formed on the surface of the rust-preventing layer. It is preferable that a specific example of the coat be an organic coat which contains an epoxy resin and a lubricating component, because rust-preventing characteristics, adhesion and lubrication can function effectively.
On the obverse surface of each of the exterior members, a rust-preventing layer made of a conversion treatment coat is formed on the metal sheet, and a coat which contains a lubricating component and aims to improve the finish (appearance and durability) of the overcoating is formed on a surface of the rust-preventing layer. It is preferable that a specific example of the coat be, for example, an organic coat which contains a polyester resin and the lubricating component, because the organic film excels in surface smoothness, weather resistance and lubrication.
Moreover, each of the exterior members may be made of an aluminum alloy excellent in rust-preventing characteristics.
Part or all of the inside members which constitute the chassis may be made of an aluminum alloy.
According to a second aspect of the present invention, there is provided a vehicle body coating method for an automobile which comprises: a step of press-forming a precoat metal sheet in which a rust-preventing layer and a coat which contains a lubricating component and-aims to improve the finish (appearance and durability) of overcoating are formed in that order on at least an outside surface of a metal sheet, thereby producing plural exterior members, such as a bonnet, a trunk lid, doors and fenders, which constitute an outside of a vehicle body; a step of press-forming a precoat metal sheet in which a rust-preventing layer which contains a lubricating component is formed on a surface of a metal sheet, thereby producing plural inside members, such as a floor panel and an inner panel, which constitute an inside of the vehicle body, and joining the inside members to produce a chassis for the automobile; a step of joining the exterior members to the chassis; and a step of performing overcoating on surfaces of the exterior members joined to the chassis.
In the second aspect of the invention, as in the first aspect thereof, it is not necessary to perform conversion treatment on the inside members and the exterior members of the vehicle body, and it is not necessary to perform electrodeposition coating required for the chassis. Specifically, it is not necessary to perform any of the step of degreasing and cleaning a press-formed metal sheet, the step of drying the metal sheet, the step of adjusting the surfaces of the metal sheet, a conversion treatment step which performs rust-preventing treatment on the surfaces, and a rust-preventing electrodeposition coating step, all of which steps have been required in the related art vehicle body coating method. In particular, since the exterior members which constitute the outside of the vehicle body need only overcoating, the coating of the exterior members can be implemented with a spray type of electrodeposition coating device, whereby coating work is made simple and coating equipment can be made small and simple. Drying and fixing equipment for overcoating can also be made small and simple.
According to a third aspect of the present invention, there is provided a vehicle body coating method for an automobile which comprises: a step of press-forming a precoat metal sheet in which a rust-preventing layer and a coat which contains a lubricating component are formed on at least an outside surface of a metal sheet, thereby producing plural exterior members, such as a bonnet, a trunk lid, doors and fenders, which constitute an outside of a vehicle body; a step of forming plural chassis constituent members from sheet materials, profiles or castings, each made of an aluminum alloy, and joining the chassis constituent members to produce a chassis for the automobile; a step of joining the exterior members to the chassis; and a step of performing intermediate coating and overcoating on surfaces of the exterior members joined to the chassis.
The plural sheet materials, profiles and castings which constitute the chassis are made of an aluminum alloy, and the aluminum alloy itself has rust-preventing characteristics. Accordingly, a precoat can also be omitted, but rust-preventing treatment work can easily be carried out. In other words, the conversion treatment of the inside members of the vehicle body can be omitted or simplified.
The exterior members do not need conversion treatment or electrodeposition coating, and it is not necessary to carry out electrodeposition coating which is considered to be necessary for the chassis. Since the exterior members need only intermediate coating and overcoating, coating work can be simplified.
According to a fourth aspect of the present invention, there is provided a vehicle body coating method for an automobile which comprises: a step of press-forming a precoat metal sheet in which a rust-preventing layer and a coat which contains a lubricating component and aims to improve the finish (appearance and durability) of overcoating are formed in that order on at least an outside surface of a metal sheet, thereby producing plural exterior members, such as a bonnet, a trunk lid, doors and fenders, which constitute an outside of a vehicle body; a step of forming plural chassis constituent members from sheet materials, profiles or castings, each made of an aluminum alloy, and joining the chassis constituent members to produce a chassis for the automobile; a step of joining the exterior members to the chassis; and a step of performing overcoating on surfaces of the exterior members joined to the chassis.
Although the effects and advantages of the fourth aspect of the invention are approximately the same as those of the third aspect of the invention, the exterior members whose obverse surfaces respectively have the coats aiming to improve the finish of overcoating, so that the exterior members need only overcoating and coating work can be simplified.